In a most general sense, all the research projects that my students
and I have worked on, and are planning to work on in the future, involve
normal faulting and crustal extension processes. Within this general framework,
there are several specific topics that we have focused on over the last
several years. These include: a research program that has made a significant
contribution to a fundamental understanding of the normal fault growth
process, a research program which has made significant progress toward
understanding the relationship between mantle hotspot plumes and extension
of the continental lithosphere, the study of faults zones in the brittle
crust and what can be learned about faulting at the macroscale from microscale
analysis, and a research program addressing the mechanical paradox of upper
crustal low-angle normal faulting.

Our emphasis is field based studies. In this work we use a wide variety
of techniques some of which include; paleomagnetic analysis, isotopic dating
(especially Ar/Ar), seismic reflection interpretation, thinsection microscopy,
microprobe analysis and mechanical and thermal modeling.

This project, involving Chris Scholz, myself and
a number of students, was an investigation of the scaling laws that govern
faulting and of the physical mechanisms for fault growth that gives rise
to these scaling laws. In several publications we have shown how knowledge
of the scaling laws would allow the determination of strain from faulting
in a sparse geological data set. Our work on the growth of faults was extended
to the growth of normal faults and the understanding of how the scaling
laws apply to such structures. We have concentrated on the nature of fracture
near fault zones to test our fault growth models. We have also developed
a theory of fault growth that explains how large normal faults grow and
how such a growth history can be applied to an understanding of normal
fault segmentation. This research project has resulted in four completed
Ph.D.

Schlische, R.W. and Anders, M.H., 1996, Stratigraphic
effects and tectonic implications of the growth of normal faults and extensional
basins, in Reconstructing the history of Basin and Range extension using
sedimentology and stratigraphy, editor K. Beratan, Geological Society
of America Special Publication303, p. 183-203 .

Map
view of normal fault system in the Tablelands of California, near Bishop.
Faults were formed on a 738 ka surface formed by the Bishop Tuff (Figure
from Dawers and Anders, 1995).

Figure
showing possible modes of linkage of two normal faults growing toward each
other. In order for scaling relationships between fault length and displacement
to be maintained during linkage, displacement must be accelerated in the
linkage region to account for the increased length of the fault. One way
to do this is for a number of smaller faults to develop in the linkage
regions, the sum total of displacement on these smaller faults can increase
so that scaling relationships are maintained. (Figure from Anders and Schlische,
1994).

Displacement data from a normal fault system shown above. Note the
large number of smaller faults in the region of linkage. The displacement
on these faults when summed results in a pattern for the total fault system
that is roughly representative of the displacement pattern of each of the
smaller faults. This result is consistent with the faults within a fault
system roughly being self-similar. There is however, some flattening of
the total or summed profile with respect to the smaller faults that we
suggest results from faults growing into a mechanically weak subsurface
of volcanoclastic sediments. This can be thought of as analogous to large
crustal-scale faults that extend to a weak ductile substrate. (Figure from
Dawers and Anders, 1995).

One of the first order questions concerning
hotspots is how does the continental lithosphere respond when passing over
one of them. One of the best examples of a continental hotspot is the Yellowstone
hotspot. However, the Yellowstone hotspot appears not to fit the hotspot
model sensu stricto. In other words, the assumed track rate is significantly
different than would be expected assuming the location of the major rhyolite
calderas represents the sublithospheric position of a stationary mantle
plume. My students and myself have used the migrating extensional deformation
field caused by the thermal effects of the hotspot rather than the assumed
locations of rhyolitic volcanic calderas. Using the caldera location, several
workers have estimated the migration rate of the Yellowstone volcanism
to range from 2.9 cm/yr to 4.5 cm/yr. over intervals ranging from 10 m.y.
to 16 m.y. This is a significantly greater rate than one would expect based
on an assumed fixed plume source and plate motion based on NUVEL1 plate
motion models. The difference may be due to the inability to accurately
assess the migration rate because of the inability to determine the exact
location of calderas because they are buried by 1 to 3 km of basalt.

Using the deformation field rather than
caldera locations, we have determined an extension-corrected relative velocity
of 2.2 ± 0.2 cm/yr. This value is significantly less than the previous
extension-derived estimate of plate velocity of 3.7 cm/y.

In short, we have determined an independent
estimate of the North American plate velocity of 2.2 ± 0.2 cm/yr
and found that the many of the published caldera locations are in error
suggesting that the hotspot, for the last 10 Ma, tracks exactly as expected
for the "standard" hotspot model.

Anders, M.H., Rodgers, D.W., McCalpin, J.P.,
and Haller, K.M., 1990, Late Tertiary and Quaternary faulting north and
south of the eastern Snake River Plain, in Geologic Field Tours of Western
Wyoming and Parts of Adjacent Idaho, S. Roberts, editor, Geological
Survey of Wyoming Public Information Circular no. 29, p. 1ó3.

Figure of western U.S showing the track of the Yellowstone hotspot
as assumed by younging in a northeast direction of silicic calderas (yellow).
Areas colored in shades of tan represent Neogene sedimentary deposits contemporaneous
with the hotspot volcanism. Black dots are sampling locations of Yellowstone/Snake
River Plain volcanic ash deposits. All radiometric ages for units in the
interval including 4.49 Ma to 10.27 Ma are ages determined by Ar/Ar analysis
at the Lamont-Doherty Earth Observatory Geochronology Laboratory and the
at the Berkeley Geochronology Center. Many age determinations are
based on weighted averages of as many as 16 individual age determinations.

Map showing the seismicity in the vicinity of Yellowstone and the eastern
Snake River Plain. Three colors represent the respective positions of the
seismic parabola as assessed by several authors. Note that is an interior
parabola that defines a zone of aseismicity called the collapse shadow
(Anders, 1983). The pattern defined by Pierce and Morgan (1992) is based
primarily on the location of active normal faults whereas the two parabolas
defined by Anders et al. (1989) are based solely on the distribution of
seismicity.

Map
showing the location of faults whose interval of accelerated displacement
was determined. The accelerated extension is thought to be related
to arrival of the migrating seismic parabola. Distance is measured between
sampling location and the position of the outer parabola along a line parallel
to the migration path of the /Yellowstone/Snake River Plain volcanic center.

Plot
of the interval of accelerated extension, determined at a particular location,
versus the distance to the outer parabola alone a line trending N55°E
(from Anders, 1994). Assuming the migration direction can reasonably be
determined, the slope of the line reflects the migration rate of the thermomechanical
effects of an assumed sub-lithospheric plume or hotspot. From these data
the migration rate of the thermal source is determined to be 2.2 ±
0.2 cm/yr over the last 10 m.y. This is significantly less than the 2.9
cm/yr to 4.5 cm/yr values that have been suggested based on the location
of volcanic centers. If the thermal source is indeed fixed relative
to the North American plate, as predicted by the standard hotspot model,
then the relative North American plate velocity is 2.2 cm/yr in a roughly
N55°E direction. This estimate is the same as that calculated for the
last 3 m.y. using sources other than the Yellowstone hotspot (see DeMets
et al. 1990).

Geologists from around the world have reported
finding normal faults that they suggest moved at low-angles(< 30°)
in the brittle field. Our current understanding of fundamental rock mechanics
suggests that normal faults should not move at such low angles. Moreover,
there is no definitive evidence of a historic earthquake rupture on such
a low-angle normal fault. Absent some special conditions, either the field
evidence is incorrect or our understanding of fundamental mechanics is
somehow misguided.

We at Lamont, Nick Christie-Blick, Kate Gregory-Wodzicki
and my graduate students and myself, have undertaken an extensive reexamination
of several of the key geologic examples of low-angle normal faults that
played a significant role in advancing the concept of upper crustal low-angle
normal faulting. One of the most prominent examples of an upper crustal
low-angle normal fault is the Sevier Desert detachment of west-central
Utah. The detachment is thought to be an 11° west dipping fault that
has accommodate about 40 km of displacement with and a poorly defined length.
Despite its size, prior to our work this feature, the fault itself had
not been seen in outcrop, rather seen only on seismic reflections lines.
The existence of this feature has strongly effected the thinking on the
mechanical paradox of low-angle normal faulting. We at Lamont have come
to questioned whether this feature seen on seismic reflection profiles
is truly a fault. Our challenge of this concept is based on our examining
material from well cuttings and core from the presumed fault surface. In
examining the material from the location of the reflection, we found no
evidence of either brittle or ductile deformation from either the upper
or the lower contact which produces the prominent seismic reflection. We
have, therefore, concluded that Tertiary/Paleozoic contact thought to be
the fault surface is actually an unconformable contact. This interpretation
of course presents many challenging questions such as how did the Tertiary
Sevier Desert basin form and how might the communities interpretations
of the Mesozoic thrust system be altered?

One of the keys to understanding the faulting
process in the brittle part of the crust is by examining the fault zone.
Brittle failure produces a distinctive kind of deformation that characterizes
the movement regiem of a fault. My students and myself have studing the
growth of faults from incipinent ruptures to fault exhibiting significant
displacement. From these studies we have been able to determine the changes
in the stress field associated with the development of a process zone.
Using microfractures, we have mapped that changes in the orientation of
the principle stress axes yielding hitherto unobserved behavior that is
consistent with an elastic/inelastic model of fault zone development.

We have also examined in detail the deformation patterns
associated with shallow crustal normal fault and comparing these observation
to the deformation style of deformation found at the base of large slide
blocks. As one might expect a priori, there are significant differences.
More importantly, we have found that the unique deformation style found
at the base of landslides and slide blocks is almost identical to the deformation
style found at the base of several low-angle detachment blocks thought
to be associated with significant crustal extension.

Some of the broad research directions I view as potentially
significant include topics such as how we distinguish between mechanical
unroofing of a large mountain range and unroofing of that range by climatically
driven processes? Similar questions include how rocks thought to be formed
as much as 70 km beneath the Earth's surface make it to the Earth's surface?
When mechanical processes are involved in unroofing, what are they? As
with all of the previous research topics that my students I have addressed,
we will take a multi-disciplinary approach. Areas we have drawn on in our
past research as well as for futures endeavors include, field mapping,
isotope geochronology, paleomagnetics, mechanical modeling, petrographic
analysis to name just a few. Some of the new projects that I intend to
undertake involve diverse projects such as determining the mechanic processes
involved in opening of the Red Sea, the uplift history of the Andes, and
the origin of eclogites in the Caledonides of western Norway.

PublicationsAnders, M.H., Gregory-Wodzicki, K.M., Spiegelman, M., A critical
evaluation of late Tertiary accelerated uplift rates for the Eastern Cordillera,
central Andes of Bolivia: (in press) Journal of Geology.Back to top